Any component on a road bike can be replaced, some with smaller parts and others as a complete unit. This is good if something wears out and needs replacing or if you have a crash and a part is destroyed. It also means the opportunity to upgrade is there whether the part needs replacing or not which may not be as good when explaining to your other half the bank balance debit.
The constant evolvement of bike design creates better parts with the latest equipment featuring innovations unthinkable a few years ago. So whether it’s a transmission or brake part, handlebars or stem, saddle or seatpin, or simply bar tape then you can bet the new part will probably be better than the old part it is replacing when it was new.